It is very often desirable, in the field of electronic countermeasure warfare, to determine the frequency of a received signal as, for example, when one wishes to produce a jamming signal of the same frequency as a signal being emitted by an enemy. These emitted signals are not always merely of a single, constant frequency, but often vary in frequency in a predetermined manner. Therefore, in order to effectively jam such signals, one must accurately and quickly determine the frequency of the originally emitted signal. There have been developed in the past various forms of panoramic scanning receivers which scan the frequency spectrum of interest in search of the frequency of the emitted signal which is desired to be jammed. Obviously, such frequency scanning is relatively slow and this time lag may often prove fatal to attempts at producing effective jamming signals.
There is now currently in use a different technique for determining the unknown frequency of a received signal, this technique has the descriptive title of instantaneous frequency measurement. Equipment utilizing this well-known technique can provide frequency information in such a short period of time that highly effective jamming signals may be produced and transmitted. This instantaneous frequency measurement technique utilizes a detector having a signal delay element, i.e., a frequency sensitive phase shifter, and a radio frequency phase difference sensor. An emitted signal is received and divided into two equal components. One signal component is fed through the delay line, and the other signal component is fed to the phase sensor. The phase difference between the two components is then measured. The magnitude of the measured phase difference is related by a known amount to the unknown frequency of the received signal. Since this relationship between a frequency and its attendant phase shift in the delay line is known, the phase difference signal can be processed and then fed to a voltage controlled oscillator to produce a jamming signal having an identical frequency to that of the originally received signal.
While the instantaneous frequency measurement technique has many advantages, unfortunately, a conventional system cannot properly handle simultaneously received pulse signals. It is fairly common for many modern radars to simultaneously emit pulse signals of two or more frequencies, resulting in their arriving simultaneously at an intercept receiver. This reception of two simultaneously emitted pulse signals of different frequencies causes the instantaneous frequency measurement unit to become hopelessly confused, and it cannot even determine the frequency of either of the two signals, provided they are of relatively equal strength. One solution to this problem is to precede the instantaneous frequency measurement detector with a conventional, electrically tuned, YIG rejection filter. This YIG filter is swept at a fixed rate across the band suspected to contain a dual frequency emitter signal. By properly selecting the filter bandwidth and the sweep rate, it should be possible to see individually each frequency of the two simultaneously received signals at least once each sweep. This filter sweeping concept is similar to the sweeping heterodyne panoramic receiver and hence contains the deficiencies of these past systems. There are other problems which are also introduced in the solving of the original dual signal problem which make this sweeping filter solution less than the best possible solution.